Method of preparing silicon nitride with a high alpha-phase content

ABSTRACT

The invention relates to preparing refractory inorganic compounds, particularly, to methods of preparing silicon nitride with a high a -phase content. 
     The method of preparing silicon nitride with a high α-phase content is accomplished by means of self-propagating high-temperature synthesis. The method includes contacting the charge containing a silicon-reagent with an additive in a nitrating medium. The silicon-reagent is metallic silicon and at least one ammonium halide is used as an additive in the amount of 1-60% of the mass of metallic silicon. The synthesis is carried out at a pressure of 4-30 MPa.

FIELD OF THE INVENTION

The present invention relates to preparing refractory inorganiccompounds and, more particularly, to methods of preparing siliconnitride with a high α-phase content.

Due to unique physical and mechanical properties (high hardness,sufficient strength and wear resistance, thermal stability, extremelylow thermal expansion coefficient, inactivity in many aggressive media,low friction coefficient, low density, specific semiconducting anddielectric properties), silicon nitride finds application formanufacturing ceramic materials used for various purposes.

These construction high-temperature materials, refractories,antifriction and tool materials, special materials with a high electricstrength and stable dielectric properties within a wide range ofconditions, with heat-insulating and heat-conducting properties arewidely used in engineering, tool industry, metallurgy, rocketengineering, electronics, electrical and radio engineering, and in otherbranches of industry.

BACKGROUND OF THE INVENTION

To prepare silicon nitride powders on industrial scale, various methodsof direct synthesis have found wide application since initial materialsare readily available.

Known methods of direct synthesis of silicon nitride powder via thenitration reaction can be divided into traditional methods of furnacesynthesis widely used in industry and a promising relatively newplasmochemical method and a method of self-propagating high-temperaturesynthesis (SHS) in the combustion regime.

The method of furnace synthesis of silicon nitride powders is based onnitration of elemental silicon powder upon heating in electric furnacesin a flow of nitrogen or nitrogen-containing gas. Nitration of silicondioxide in a mixture with a reducer, mainly, carbon, is one of theversions of the known method.

Methods of silicon nitration in furnaces are predominantly two-stage andtheir accomplishment requires much time. The first stage of siliconpowder nitration is carried out at a temperature by 100°-250° C. belowthe melting point of silicon and up to the attainment of 30-40% degreeof bonding silicon to nitrogen. This stage requires from 3-5 hrs to10-20 hrs. At the second stage complete nitration is performed at1500°-1600° C.

Known in the art is the most effective and rather simple method ofpreparing silicon nitride powder with a high content of α-phase by thefurnace synthesis method. The method resides in nitration of metallicsilicon in a flow of a nitrogen-containing gas under a reduced nitrogenpartial pressure for 4-5 hrs upon heating in furnaces at 1200°-1400° C.,the nitrogen partial pressure being maintained equal to ˜0.5 atm (0.05MPa) as long as 50-60% by mass of silicon is reacted. The methodprovides the content of α-phase in the silicon nitride powder equal to97% by mass.

The use of the known method for preparing silicon nitride requires astrict control over temperature, nitrogen partial pressure, and a gasflow rate in the course of the whole nitration process in order tomaintain the thermal conditions required for the formation of α-phaseand to compensate the exothermal effect of the nitration reaction withthe aid of heat removal. Besides, this method demands rather great powerconsumption for attaining 1200°-1400° C. in the electric furnaces atwhich the nitration process takes place.

The method of plasmochemical synthesis, namely, nitration of silicon ina low-temperature nitrogen plasma, is of interest for preparingultrafine silicon nitride powders possessing good caking ability.

Known in the art is a plasmochemical method of preparing silicon nitrideby nitration of silicon in a nitrogen plasma produced by ahigh-frequency generator with the use of 98.9% pure silicon and nitrogenof a high purity (Izd. Akad. Nauk SSSR, ser. Neorganocheskie materialy,Moscow, 1979, Vol. 15, No.4. G. M. Kheidemans, Ya. P.Grabas and T. A.Miller "High-temperature synthesis of finely dispersed silicon nitride",pp. 595-598).

Silicon nitride powder prepared by the above method is a mixture of αand β phases and contains 2-4% by mass of free silicon and up to 5% bymass of oxygen. In addition, plasmochemical powders of silicon nitridepossess an enhanced chemical activity as compared with powders obtainedby other methods and are readily hydrolyzed in humid air which requirescertain measures upon storage and processing.

Thus, although plasmochemical powders possess a tendency to cake, thequality of these powders does not allow one to use them for preparingceramic materials with high physical and mechanical properties.

Besides, the accomplishment of the plasmochemical synthesis, as well asthat of the furnace synthesis, demands great power consumption.

The method of self-propagating high-temperature synthesis (in thecombustion regime) (U.S. Pat. A, No. 3726643) is the most promising onefor direct synthesis of silicon nitride with respect to the purity andquality of the prepared product, efficiency and energy-intensity of theprocess.

The method is based on the use of heat liberated upon exothermalinteraction of the reagents at least one of which is in a condensedstate. The method resides in local initiation of a chemical reaction ina layer of the reaction mixture and in a subsequent interaction of thereagents in the combustion regime, i.e. self-propagation of thecombustion front at the expense of layer-by-layer self-heating of thereaction mixture due to sufficient exothermal effect of the reaction.

A great thermal effect of the reaction of silicon with nitrogen (180kcal/mole) makes it possible to carry out the process of siliconnitration in the combustion mode, i.e. by the method of self-propagatinghigh-temperature synthesis (SHS).

Moreover, the exothermic effect of the reaction of silicon with nitrogenis so high that the combustion temperature must be decreased in order tomaintain the temperature preferable for the formation of α-phase ofsilicon nitride.

To decrease the combustion temperature, the initial powderous mixture ofthe reagents is diluted to 50% by mass with the final product.

Silicon nitride with a high content of α-phase was prepared by themethod of self-propagating high-temperature synthesis with dilution ofthe initial powderous mixture by the final product (J.Am. Ceram. Soc.,1986, Vol. 69, No. 4; Kiyoshi Hirao, Yoshinary Miyamoto, Mitsue Koizume"Synthesis of silicon nitride by a combustion reaction under highnitrogen pressure", pp. 60-61).

The method resides in preparing an initial powderous mixture (charge)from silicon powder (99.9% pure) with a dispersity of ≲5 mkm and siliconα-nitride powder (98% of α-Si₃ N₄) with a dispersity of ˜0.1 mkm at amass ratio of 47.4 and 52.6% respectively, grinding the chargecomponents to a dispersity of ˜0.2 mkm, mixing in acetone, drying thecharge in a vacuum, molding the cylinders 6 mm in diameter and 10 mmlong with a density of 44-46% of theory, and performing the synthesis inthe combustion mode under 10 MPa after local initiation of the reactionin a charge layer with a 3 s electric pulse.

The synthesis in the combustion mode yielded silicon nitride with 87% bymass of α-phase.

Thus, the preparation of silicon nitride with a high content of α-phaserequires the use of silicon nitride with almost a 100% content ofα-phase in amount of 50% of the initial charge mass as an inertcomponent in making the initial charge.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of preparingsilicon nitride with a high α-phase content which will ensure thepreparation of the final product without using scarce material.

This object is accomplished by a method of preparing silicon nitridewith a high α-phase content using self-propagating high-temperaturesynthesis including the contact of the charge containing silicon-reagentand an additive with a nitrating medium under an increased pressure.According to the invention, metallic silicon is used as silicon-reagentand as an additive use is made of at least one ammonium halides inamount of 1-60% of the mass of silicon-reagent and self-propagatinghigh-temperature synthesis is carried out under a pressure of 4-30 MPa.The pressure of the nitrating medium below 4 MPa results in a decreaseof the silicon nitride yield due to the reduction of the nitrationdegree of silicon.

The pressure of the nitrating medium above 30 MPa is inexpedient since afurther pressure rise does not further increase the silicon nitrationdegree.

The additives to the charge proposed in the method plays a double rolein the course of the synthesis. Firstly, they are the diluting agent ofthe charge, i.e. they decrease the combustion temperature, therebyproviding a temperature regime favourable for the formation of α-phase.

If the content of the additives is less than 1% by mass there is a riseof the temperature of combustion and dissociation of the final product,i.e. increases the lower critical combustion limit.

If the content of the additives is more than 60% by mass there is anincrease in the upper critical combustion limit in the given system(i.e. the combustion becomes impossible).

Secondly, these additives are not inert in the process ofself-propagating high-temperature synthesis. Being gasified in thecombustion front, they form with silicon intermediate products whichlater favour the formation of silicon α-nitride in the recombustionregion.

For this purpose, an ammonium halide or a mixture of ammonium halides isused as an additive to the charge.

Ammonium fluoride, chloride or iodide, or mixtures thereof in any ratioscan be used as the ammonium halide.

It is recommended to introduce additional halides of Group I-III metalsinto the charge at a mass ratio of ammonium halide to metal halide equalto 1:0.01-1, respectively.

An additional introduction into the charge of halides of the Group I-IIImetals as an additive provides the preparation of silicon α-nitride witha high caking ability.

To enhance the content of silicon α-nitride in the final product, it isexpedient to introduce into the charge additionally amorphous siliconand/or silicon imide as silicon-reagent in amount of 5-95% of the massof metallic silicon, the amounts of amorphous silicon and silicon imidein the case of simultaneous use thereof being within the range fromidentical to a 10-fold excess of one component with respect to theother.

The introduction of amorphous silicon, as well as of silicon imide,enhances the reactivity of silicon-reagent which ensures theaccomplishment of synthesis in the combustion mode at a lowertemperature.

The content of amorphous silicon and/or silicon imide less than 5% bymass does not ensure a high content of α-phase in the final product. Thecontent of amorphous silicon and/or silicon imide more than 95% by massis inexpedient since the yield of the final product does not increasefurther.

When amorphous silicon and silicon imide are used simultaneously, theirmass ratio from equal amounts to a 10-fold excess of one component withrespect to the other is optimum.

To decrease the combustion temperature in the process ofself-propagating high-temperature synthesis, it is expedient to carryout the process in the flow of the nitrating medium aftersilicon-reagent has reacted by 40-60%. This limit is optimum foraccomplishment of self-propagating high-temperature synthesis in theflow of the nitrating medium.

It is expedient to use ammonia, nitrogen, nitrogen in a mixture with1-30 vol. % ammonia, hydrogen, halide, hydrogen halide, and argonseparately or in combinations with one another as a nitrating medium.

Self-propagating high-temperature synthesis can be accomplished withliquid nitrogen as a nitrating medium.

It is recommended, in order to enhance the yield of the final product,to subject metallic silicon to preliminary chemical or physical action.As a result, the reactivity of silicon increases. It also allows one toperform synthesis at lower temperature.

Metallic silicon can be preliminary treated with hydrofluoric acid. Thetreatment is carried out in a fluoroplastic vessel with the use ofdiluted hydrofluoric acid upon stirring.

Metallic silicon can also be preliminary treated with ultrasound,shock-wave effect, or vibration dispersion by following the traditionalprocedures.

Treatment of metallic silicon with ultrasound in the action of anultrasound field on powderous silicon.

Treatment by a shock-wave effect consists in the action of a shock wavegenerated by the explosion of an explosive substance on anindestructible capsule containing powderous silicon.

Treatment of metallic silicon by vibration dispersion resides inscreening a silicon powder through the layer of vibrating balls in acrusher of the desintegrator type.

The proposed method of preparing silicon nitride with a high α-phasecontent ensures the preparation of the final product with the content ofα-phase up to 95% having a good caking ability and low shrinkage uponcaking.

The proposed method does not require complex equipment, scarce rawmaterial, and great energy consumption. The initial electric power forinitiation of the chemical reaction in a charge layer with an electricpulse is negligibly small.

DETAILED DESCRIPTION OF THE INVENTION

The method of preparing silicon nitride with a high α-phase content istechnologically simple and is realized in the following way.

A charge is prepared by mixing the initial components in a ball mill for20 min.

A powder of metallic silicon with a dispersity of 10 mkm and ammoniumhalide or a mixture of ammonium halides are mixed in the ball mill.

The obtained charge is loaded into a reactor onto a high-temperatureporous substrate. The reactor represents a high pressure vessel made ofstainless steel with a water jacket for cooling.

The reactor is filled with gaseous ammonia or nitrogen, or nitrogen in amixture with 1-30 vol. % of ammonia, hydrogen, halide, hydrogen halide,and argon used separately or in a combination with one another under apressure of 4-30 MPa.

Then the initial initiation of the chemical reaction is carried out in asmall layer of the charge. For this purpose, a short pulse (5-10 s) of20-50 V electric current (30-70 A) is delivered to an electric coil madeof tungsten wire 25 mm in diameter. A hot coil heats the charge layer incontact up to high temperature at which the interaction of silicon withnitrogen begins and then the chemical reaction propagates in the chargein a layer-by-layer combustion mode at a propagation rate of thecombustion wave 0.5-2 mm/s in the given system (depending on thecomposition of the charge and nitrating medium).

The additives are gasified in the combustion front which favours, on theone hand, a decrease of the combustion temperature and, on the otherhand, the formation of the intermediate reaction products producingsilicon α-nitride in the combustion zone. After the completion of thesynthesis and cooling the reactor, the pressure in the reactor isreleased and the final product is discharged in the form of a cake. Theouter layer of the cake is cleaned from the products of incompleteburning.

When amorphous silicon and/or silicon imide and halides of Group I-IIImetal are used, they are introduced into the charge at the stage ofpreparing thereof upon mixing the initial components in a ball mill.

Self-propagating high-temperature synthesis of silicon nitride in anitrating medium flow is accomplished as follows.

The prepared charge is loaded into the reactor, the reactor is filledwith a nitrating gas under a pressure of 4-30 MPa, and the reaction isinitiated with an electric pulse as described above. After 40-60% ofsilicon-reagent has reacted, i.e. the reaction is almost half completed,the nitrating gas begins to be blown-off at a chosen flow rate.

The combustion reaction process is controlled by the reaction time whichcan be calculated by the known rate of combustion in the given system.Optimum flow rate of the nitrating medium is 4-8 l/min.

After completion of the synthesis and cooling the reactor, the pressureis released to atmospheric and the final product is discharged.

If liquid nitrogen is used as a nitrating medium, the combustionreaction proceeds under nitrogen pressure arising in the charge poresdue to a high temperature because of the exothermal effect of thereaction.

The process is accomplished in the following way.

The prepared charge is placed onto a high-temperature substrate andloaded into a reactor made as a Dewar flask. Then the reactor is filledwith liquid nitrogen and the chemical reaction is initiated with anelectric pulse.

After the completion of the reaction and cooling the reactor, the latteris opened and the final product is discharged.

For a better understanding of the present invention, specific examplesare given hereinbelow by way of illustration.

The content of α-phase in silicon nitride was determined by X-rayanalysis and the content of free silicon by chemical analysis (byvolumetric determination of hydrogen displaced from an alkali solutionby silicon).

EXAMPLE 1

A charge is prepared from a powder of metallic silicon and an additiveof ammonium fluoride in amounts of 60% by mass of the silicon mass.

To prepare a charge, 3,000 g metallic silicon and 1,800 g ammoniumfluoride are mixed in a ball mill within a time period of 20 min. Thecharge thus obtained is loaded into a reactor onto a poroushigh-temperature substrate. The reactor is filled with gaseous nitrogenunder a pressure of 10 MPa. The chemical reaction is initiated bydelivering for 5-10 s an electric pulse (30-70 A, 20-50 V) to anelectric coil made of a tungsten wire 25 mm in diameter. The hot coilheats the charge layer in contact up to the temperature at which theinteraction of silicon with nitrogen with heat liberation begins. Thenthe chemical reaction proceeds in a self-propagating mode oflayer-by-layer combustion. After the completion of the combustionreaction and cooling the reactor, the pressure in the reactor isreleased to atmospheric and the final product is discharged. The outerlayer is cleaned from the products of incomplete combustion. The contentof α-phase in silicon nitride is 88% by mass and the content of freesilicon is 1% by mass.

EXAMPLE 2

A charge is prepared from 95% by mass of metallic silicon and 5% by massof amorphous silicon with an additive taken in amount 60% ofsilicon-reagent. Ammonium fluoride is used as the additive.

The charge prepared by mixing the components is loaded into a reactorwhich is filled with nitrogen up to a pressure of 10 MPa and thechemical reaction is initiated. Then the process is carried out asdescribed in Example 1.

The content of α-phase in silicon nitride is 89% by mass, the content offree silicon is 0.2% by mass.

EXAMPLE 3

A charge is prepared from 95% by mass of metallic silicon and 5% by massof silicon imide with an additive taken in amount 40% of the mass ofsilicon-reagent. A mixture of ammonium fluoride and chloride at a massratio 1:1 is used as the additive. Aluminium fluoride and a mixture ofammonium halides at a mass ratio 1:0.01 are also used as an additive.

The charge prepared by mixing the powders of metallic silicon, siliconimide, ammonium fluoride, ammonium chloride and aluminium fluoride isloaded into a reactor, the reactor is filled with nitrogen under apressure of 15 MPa, and then the process is performed by following theprocedure described in Example 1.

The content of α-phase in silicon nitride is 93% by mass and the contentof free silicon is 0.07% by mass.

EXAMPLE 4

A charge is prepared from 5% by mass of metallic silicon and 95% by massof amorphous silicon with an additive in amount of 1% of the mass ofsilicon-reagent. A mixture of ammonium fluoride and chloride at a massratio thereof equal to 1:1 is used as the additive.

The charge obtained by mixing the powders of metallic silicon, amorphoussilicon, ammonium fluoride and ammonium chloride is loaded into thereactor. The reactor is filled with nitrogen in a mixture with 30 vol. %of argon under a pressure of 30 MPa. Then the process is carried out byfollowing the procedure described in Example 1.

The content of α-phase in silicon nitride is 88% by mass and that offree silicon is 1% by mass.

EXAMPLE 5

A charge is prepared by mixing 45% by mass of metallic silicon, 50% bymass of amorphous silicon and 5% by mass of silicon imide with anadditive taken in amount of 40% of the mass of silicon-reagent. As theadditive use is made of ammonium fluoride and chloride at a mass ratiothereof 1:0.1.

The obtained charge is loaded into a reactor and the reactor is filledwith nitrogen in a mixture with 10 vol. % of ammonia under a pressure of12 MPa. Then the process is performed as described in Example 1.

The content of α-phase in silicon nitride is 95% by mass and that offree silicon is 0.15% by mass.

EXAMPLE 6

A charge is prepared from 45% by mass of metallic silicon, 5% by mass ofamorphous silicon, 50% by mass of silicon imide with an additive takenin amount of 5% of the silicon-reagent mass. As the additive use is madeof a mixture of ammonium fluoride and chloride at a mass ratio thereofequal to 2:1. Sodium fluoride is also used as an additive at a massratio ammonium halides: sodium fluoride equal to 1:0.1. The preparedcharge is loaded into a reactor, the reactor is filled with nitrogen ina mixture with 5 vol. % of hydrogen under a pressure of 10 MPa. Then theprocess is carried out by following the procedure described in Example1.

The content of α-phase in silicon nitride is 95% by mass and of freesilicon is 0.06% by mass.

EXAMPLE 7

A charge is prepared from 90% by mass of metallic silicon, 10% by massof amorphous silicon, and an additive taken in amount of 25% of thesilicon-reagent mass. As the additive use is made of a mixture ofammonium fluoride and chloride at a mass ratio thereof 1:1.

The prepared charge is loaded into a reactor and the reactor is filledwith nitrogen under a pressure of 4 MPa; then the chemical reaction isinitiated.

After the combustion front propagates by 40% in the charge, i.e. in 15min after the beginning of combustion, the reactor is blown-off withnitrogen at a flow rate of 6 1/min. After the completion of thecombustion process, the procedure described in Example 1 is followed.

The content of α-phase in silicon nitride is 94% by mass and of freesilicon is 0.1% by mass.

EXAMPLE 8

A charge with the composition similar to that described in Example 7 isprepared and loaded into a reactor. The reactor is filled with nitrogenunder a pressure of 4 MPa and the chemical reaction is initiated.

After the propagation of the combustion front in the charge by 60%, i.e.in 20 min after the beginning of the combustion, the reactor isblown-off with nitrogen at a flow rate of 6 1/min. After the completionof the combustion process, the procedure described in Example 1 isfollowed.

The content of α-phase in silicon nitride is 94% by mass and that offree silicon, 0.15% by mass.

EXAMPLE 9

A charge is prepared from 95% by mass of metallic silicon, 2.5% by massof amorphous silicon, 2.5% by mass of silicon imide, and an additive inamount of 30% of the mass of silicon-reagent. A mixture of ammoniumfluoride and chloride at a mass ratio thereof equal to 10:1 is used asthe additive.

The prepared charge is loaded into a reactor, the reactor is filled withnitrogen in a mixture with 10 vol. % of hydrogen chloride under apressure of 20 MPa, and then the process is performed by following theprocedure described in Example 1.

The content of α-phase in silicon nitride is 93% by mass and of freesilicon, 0.07% by mass.

EXAMPLE 10

A charge is prepared from 70% by mass of metallic silicon, 30% by massof amorphous silicon, and an additive taken in amount of 30% of the massof silicon-reagent. As the additive use is made of a mixture of ammoniumfluoride, chloride and iodide at a mass ratio thereof equal to 1:1:1.

The prepared charge is loaded into a reactor and the reactor is filledwith nitrogen under a pressure of 15 MPa. Then, follows the proceduredescribed in Example 1.

The content of α-phase in silicon nitride is 95% by mass and of freesilicon, 0.07% by mass.

EXAMPLE 11

A charge is prepared from 50% by mass of metallic silicon, 50% by massof amorphous silicon, and an additive in amount of 20% of the mass ofsilicon-reagent. A mixture of ammonium fluoride and chloride is used asthe additive at a mass ratio thereof equal to 1:1. Magnesium fluoride ata mass ratio of ammonium halides: magnesium fluoride equal to 1:0.1 isalso used as the additive.

The obtained charge is loaded into a reactor, and the reactor is filledwith ammonia under a pressure of 8 MPa. Then the process is performed byfollowing the procedure described in Example 1.

The content of α-phase in silicon nitride is 95% by mass and of freesilicon, 0.06% by mass.

EXAMPLE 12

A charge is prepared from 60% by mass of metallic silicon, 40% by massof amorphous silicon, and an additive in amount of 15% of the mass ofsilicon-reagent. As the additive use is made of a mixture of ammoniumchloride and fluoride at a mass ratio thereof equal to 1:1.

The prepared charge is loaded into a reactor, and the reactor is filledwith nitrogen in a mixture with 5 vol. % of hydrogen and 20 vol. % ofargon under a pressure of 20 MPa. Then the process is carried out byfollowing the procedure described in Example 1.

The content of α-phase in silicon nitride is 93% by mass and of freesilicon, 0.15% by mass.

EXAMPLE 13

A charge is prepared from 70% by mass of metallic silicon, 30% by massof amorphous silicon, and an additive in amount of 35% of the mass ofsilicon-reagent. As the additive use is made of a mixture of ammoniumfluoride and calcium fluoride at a mass ratio thereof equal to 1:0.1.

The prepared charge is loaded into a reactor, the reactor is filled withnitrogen in a mixture with 1 vol. % of chloride under a pressure of 15MPa, and then the process is carried out as described in Example 1.

The content of α-phase in silicon nitride is 94% by mass and of freesilicon, 0.06% by mass.

EXAMPLE 14

A charge is prepared from 80% by mass of metallic silicon and 20% bymass of amorphous silicon with an additive in amount of 35% of the massof silicon-reagent. As the additive use is made of a mixture of ammoniumfluoride and chloride at a mass ratio thereof equal to 1:1.

The prepared charge is loaded into a reactor, and the reactor is filledwith liquid nitrogen. Then the chemical reaction in a charge layer isinitiated with an electric pulse by following the procedure described inExample 1. A nitrogen pressure up to 30 MPa is generated in the chargepores because of a high temperature caused by the exothermal effect ofthe reaction and then the reaction propagates in the charge in aself-propagating combustion mode.

After the completion of the combustion process, the reaction product isdischarged.

The content of α-phase in silicon nitride is 90% by mass and of freesilicon, 0.1% by mass.

EXAMPLE 15

A charge is prepared from 80% by mass of metallic silicon treated withfluoric acid, 20% by mass of amorphous silicon, and an additive inamount of 35% of the mass of silicon-reagent. As the additive use ismade of a mixture of ammonium chloride and iodide at a mass ratiothereof equal to 10:1.

The prepared charge is loaded into a reactor, and the reactor is filledwith nitrogen under a pressure of 15 MPa. Then the process is performedas described in Example 1.

The content of α-phase in silicon nitride is 91% by mass and of freesilicon, 0.1% by mass.

EXAMPLE 16

A charge is prepared from 80% by mass of metallic silicon treated by theshock-wave effect, 20% by mass of amorphous silicon, and an additive inamount of 35% of the mass of silicon-reagent. As the additive use ismade of a mixture of ammonium fluoride and iodide at a mass ratiothereof equal to 10:1, respectively.

The prepared charge is loaded into a reactor, and then the process iscarried out as described in Example 1.

The content of α-phase in silicon nitride is 91% by mass and of freesilicon, 0.15% by mass.

EXAMPLE 17

A charge is prepared from 80% by mass of metallic silicon treated withultrasound, 20% by mass of amorphous silicon, and an additive in amountof 35% of the mass of silicon-reagent. Ammonium chloride is used as theadditive.

The prepared charge is loaded into a reactor, and the reactor is filledwith nitrogen in a mixture with 5 vol. % of hydrogen under a pressure of15 MPa. Then the process is carried out as described in Example 1.

The content of α-phase in silicon nitride is 92% by mass and of freesilicon, 0.1% by mass.

EXAMPLE 18

A charge is prepared from 80% by mass of metallic silicon treated in avibrating crusher by vibration dispersion and 20% by mass of amorphoussilicon with an additive in amount of 35% of the mass ofsilicon-reagent. Ammonium fluoride is used as the additive.

The prepared charge is loaded into a reactor, the reactor is filled withnitrogen in a mixture with 5 vol. % of HCl under a pressure of 15 MPa.Then the process is carried out as described in Example 1.

The content of α-phase in silicon nitride is 92% by mass and of freesilicon, 0.08% by mass.

EXAMPLE 19

A charge is prepared from 5% by mass of metallic silicon, 50% by mass ofamorphous silicon, 45% by mass of silicon imide, and an additive ofammonium fluoride in amount of 5% of the mass of silicon-reagent.

The prepared charge is loaded into a reactor, and the reactor is filledwith nitrogen under a pressure of 15 MPa. Then the process is carriedout as described in Example 1.

The content of α-phase in silicon nitride is 95% by mass and of freesilicon, 0.05% by mass.

INDUSTRIAL APPLICABILITY

The proposed method of preparing silicon nitride with a high alpha-phasecontent may find application in powder metallurgy for manufacturingconstruction high-temperature ceramics (for instance, engine parts),instrumental ceramics, antifriction and special materials possessing ahigh electric strength and stability of dielectric characteristics.

We claim:
 1. In a self-propagating high-temperature synthesis forpreparing silicon nitride from a charge containing a silicon reagent,the improvement comprising employing metallic silicon as the siliconreagent, contacting the metallic silicon with 1-60% of at least oneammonium halide based on the mass of the metallic silicon in a nitratingmedium at a pressure of 4-30 MPa.
 2. The method of claim 1 wherein thecharge further comprises amorphous silicon, silicon imide or combinationthereof in an amount of 5-95% based on the mass of the metallic silicon.3. The method of claim 1 wherein the charge contains amorphous siliconand silicon imide in amounts such that the weight of one does not exceedten times the weight of the other.
 4. The method of claim 1 wherein thecharge further comprises a halide of a Group I-III metal at a mass ratioof ammonium halide to said Group I-III metal halide in the range of1:0.01-1.
 5. The method of claim 1 comprising conducting the reaction ina flow of the nitrating medium after the consumption of 40-60% of thesilicon reagent.
 6. The method of claim 1 wherein the nitrating mediumis ammonia.
 7. The method of claim 1 wherein the nitrating medium isnitrogen.
 8. The method of claim 7 wherein the nitrating medium includes1-30 volume % of at least one substance selected from the groupconsisting of ammonia, hydrogen, halide, hydrogen halide and argon. 9.The method of claim 7 wherein the nitrogen is liq nitrogen.
 10. Themethod of claim 1 further comprising activating the metallic silicon byreaction with hydrofluoric acid prior to contacting the metallic siliconwith the ammonium halide.
 11. The method of claim 1 comprisingactivating the metallic silicon with ultrasound prior to contacting themetallic silicon with the ammonium halide.
 12. The method of claim 1comprising activating the metallic silicon by a shock-wave effect priorto contacting the metallic silicon with the ammonium halide.
 13. Themethod of claim 1 comprising activating the metallic silicon byvibration dispersion prior to contacting the metallic silicon with theammonium halide.